1. Field of the Invention
The present invention relates to an image display device, and more particularly, to an IPS (in-plane switching) mode LCD.
2. Description of the Prior Art
An image display device is an optoelectronic device able to transfer electric signals into visible images so that human beings can see the information contained in the electronic signals. These days, the typical image display device is commonly an LCD, and other display devices, such as a PDP (plasma display panel) display, an EL (electro luminescence) display, an FED (field emission display), and a DMD (deformable mirror device) display.
Among all those image display devices, an active matrix LCD, which utilizes thin film transistors as switching elements, is popular due to its small size, light weight, and low power consumption. The LCD substantially includes two glass substrates, a liquid crystal layer disposed between the two substrates, and two alignment films respectively disposed on two opposite surfaces of the two substrates for orientating liquid crystal molecules in predetermined directions. The liquid crystal molecules arranged in predetermined directions are rotated when an electric filed is applied, and thus images are displayed via light transmittance changes through the liquid crystal layer.
For known art, a TN (twisted nematic) mode LCD has been in use for a long time. In the TN mode LCD, the liquid crystal molecules are rotated in a vertical direction with respect to the substrate. However, this leads to problems of narrow viewing angles and color distortions.
In order to solve these problems, Japanese patent (publication No. 07-36058) has proposed an IPS mode LCD. In the Japanese patent, the IPS mode LCD includes a plurality of pixel electrodes and a plurality of common electrodes arranged in parallel to the pixel electrodes in an array substrate. A voltage is applied between the pixel electrode and the common electrode so that a parallel electric field, which rotates the liquid crystal molecules, is generated above the array substrate. Since the liquid crystal molecules are rotated in a plane approximately parallel to the array substrate, the problems of color distortions and narrow viewing angles are reduced. Therefore, the IPS mode LCD is suitable for use in large-sized displays.
For the IPS mode LCD, however, an uneven brightness defect occurs when a large-sized screen with fine image qualities is desired. Presumably, the uneven brightness defect results from the existence of impurity ions. FIG. 14 is a schematic diagram illustrating an image display condition of a conventional LCD. As shown in FIG. 14, when a white image is displayed in a display region 102, red color unevenness defects 104 appears around the display region 102. These red color unevenness defects 104 degrade the image quality of the IPS mode LCD.